1. Field of the Invention
The present invention generally relates to methods for fabricating field effect transistors, and more particularly, to methods for fabricating filed effect transistors to improve schottky contact characteristics having a major effect on electrical characteristics of field effect transistors using nitride semiconductors.
2. Discussion of Related Art
A field effect transistor using a nitride semiconductor having a high bandgap, there is a high electron mobility transistor (HEMT), a heterojunction field effect transistor (HFET), etc. Such a field effect transistor, as shown in FIG. 1, is composed of: first and second semiconductor layers 2 and 3 having a different bandgap from each other to form a heterojunction and deposited on a substrate 1 in sequence; source 4a and drain 4b forming an ohmic contact between the second semiconductor layer 3 used as a device region and a metal; and a gate 5 forming a schottky contact between the second semiconductor layer 3 and the metal.
When positive voltage is applied to the drain 4b against the source 4a, the electric field created between the drain 4b and the source 4a causes electrons to move from the source 4a to the drain 4b. At this time, the amount of electric current flowing through a channel is adjusted depending on the thickness of a depletion layer formed according to the voltage applied to the gate 5. For this rectifying action, the gate 5 needs the schottky contact. Thus, a schottky barrier, a backward characteristic, and a leakage current characteristic act as major factors to improve an HEMT device, particularly, amplifying efficiency, power performance, and frequency characteristic of an HEMT power amplifier.
The nitride semiconductor has large bandgap energy and causes the Fermi level pinning phenomenon that a Fermi level is pinned to a predetermined level, when it contacts a metal. Therefore, in forming the schottky contact having a high barrier using the nitride semiconductor, 1) the Fermi level is pinned after performing the surface treatment to reduce a surface defect of the semiconductor causing the Fermi level pining phenomenon, or 2) a high work function metal is used. Generally, as the metal used for the schottky contact, there are platinum (Pt), nickel (Ni), ruthenium (Ru), iridium (Ir), etc. However, such metal still has only a low schottky barrier but also needs a subsequent heat treatment process.
As a conventional deposition method, an electron-beam method, a heating method, or a sputtering method has generally been used to form a metal contact. In the case where such deposition methods are used to deposit a metal on the surface of the semiconductor, an electric current is used for evaporating the metal. At this time, the electric current generates heat, thereby increasing the temperature of the semiconductor substrate. However, when the metal is deposited on the semiconductor surface at a room temperature or beyond, the metal not only contacts the semiconductor surface but also changes the states of the semiconductor surface, thereby creating defects. Such defects act as a cause of the Fermi level pining phenomenon. Furthermore, during the heat treatment process performed after the metal deposition, the doping concentration of the semiconductor surface can be changed by reaction between the metal and the semiconductor, and a non-uniform contact surface is formed by the unevenly concentrated metal. The changed doping concentration and the non-uniform contact surface act as a cause for generating other defects on the semiconductor surface to have, thereby lowering the schottky barrier height.